(1) Field of the Invention
The present invention relates to a liquid crystal display device, and in particular, to an in-plane switching mode liquid crystal display device where an electrical field is applied parallel to the surface of the substrates.
(2) Description of the Related Art
In in-plane switching (IPS) mode liquid crystal display devices, liquid crystal molecules are aligned parallel to the surface of the panel and an electrical field (lateral electrical field) is applied parallel to the surface of the panel so that the liquid crystal molecules rotate by 90° in the plane. In such IPS mode liquid crystal display devices, a common electrode is formed on the first substrate where video signal lines (drain lines), scan signal lines (gate lines), thin film transistors and pixel electrodes are formed, and thus, the liquid crystal layer is operated through an electrical field in the plane of the first substrate, which is generated by the difference in the voltages applied to the pixel electrodes and the common electrode. In IPS mode liquid crystal display devices having this structure, pixel electrodes in linear form are formed so as to overlap the common electrode in sheet form of a transparent conductive film in a layer above the common electrode with an insulating film in between. As a result, liquid crystal molecules incline relative to the surface of the panel instead of being parallel thereto in the layer above the electrodes in linear form and in the portions between adjacent electrodes in linear form due to the electrical field generated in the direction of the normal of the first substrate, and thus, it is known that this causes the efficiency of the display mode to lower.
An example of a method for increasing this efficiency of the display mode is used in the liquid crystal display device in JP9-258265A. This liquid crystal display device has such a structure that protrusions are formed of an interlayer insulating film on the first substrate where thin film transistors are formed on the liquid crystal side, and a pixel electrode and a common electrode (counter electrode) are formed for each pixel so as to cover the surface of the protrusions. In particular, the structure provides protrusions along a pair of sides of each pixel that face each other and at the center between them with conductive films covering the protrusions along the pair of sides as pixel electrodes and with a conductive film covering the protrusion at the center as being a common electrode. Furthermore, the structure provides video signal lines in a layer beneath the pixel electrodes, that is to say, in a layer beneath the interlayer insulating film on which the pixel electrodes are formed.